Recent new-type secondary batteries such as nickel/hydride battery and Li ion secondary battery are characterized by the high energy density and because of this characteristic, these are used by mounting it on a small-size portable equipment and abruptly making a drastic growth in recent years. Particularly, the Li ion battery is predominating in the secondary batteries because the instrument using this battery can be more reduced in the weight, size and thickness.
A large number of studies are being made, for example, on a lithium ion battery using an organic electrolytic solution, where a metal oxide or a metal sulfide, such as LiCoO2, LiNiO2, LiMn2O4 or MOS2, is used for the positive electrode and lithium, a lithium alloy or a carbon material or inorganic compound capable of occluding/releasing lithium ion is used for the negative electrode. A lithium battery using LiMn2O4 or LiNiO2 for the positive electrode is reported in J. Electrochem, Soc., Vol. 138, No. 3, page 665 (1991).
Also, there are many reports on batteries using an electrically conductive polymer as an electroactive material. For example, a lithium secondary battery using a polyaniline for the positive electrode is commercially available as a coin-type battery for backup power sources. It is also reported that polyaniline can be oxidized/reduced by a proton and is applicable as a positive electrode active material of a battery using an acidic aqueous solution (see, e.g., Bull. Chem. Soc. Jpn., 57, page 2254 (1984)).
However, these lithium batteries have a problem in the safety and reliability in the case of short circuiting, high temperature, liquid leakage and unsealing because lithium and/or a lithium-base compound used therein is active to water or air and is oxidized easily. Therefore, various methods for improving the safety are employed, such as an improvement of the separator, incorporation of a PTC element or sealing.
For the purpose of improving the safety, high-speed current characteristics and the like which are defects of the above-described new-type batteries such as lithium ion battery, the present inventors have previously proposed a proton migration type secondary battery excellent in safety, reliability and current characteristics and having a long life and a high capacity (JP-A-10-289617 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). The electroactive material used in this proton migration type secondary battery is a polypyridine series and/or polypyrimidine series polymer, a sulfonic acid side chain series and/or hydroquinone series polymer, a manganese oxide and the like.
These electroactive materials are facilitated in the insertion/release of a proton and therefore, a secondary battery excellent in the safety and high-speed current characteristics can be obtained. However, a secondary battery thus obtained has much room for further improvement in its capacity for the insertion/release of a proton and the energy density greatly inferior to conventional new-type batteries.
J. Electrochem. Soc., Vol. 145, No. 4, page 1193 (1998) has reported that polyphenylquinoxaline exhibits an oxidation-reduction reaction in an acid aqueous solution.
European Patent Publication EP1035603A1 and JP-A-2000-260423 have proposed an electrode using a quinoxaline-base resin compounded with a specific sulfonic acid and applications thereof to a battery and a capacitor. Polymer having a quinoxaline structural unit, for its properties, is much more expected to contribute to improvement of the capacity as compared with conventionally used polypyridine or polyaniline.
Furthermore, JP-A-2000-30710 (U.S. Pat. No. 6,274,268) has proposed, as a means to enhance the capacity, a polymer battery where a polymer-carbon composite composition obtained by coating the surface of powdery carbon with an organic compound polymer capable of electrochemically adsorbing/desorbing a proton, such as polyaniline or polypyridine, is used as an electroactive material.
In JP-A-2001-110423 (EP 1091434A1), the present inventors have proposed use of a polymer having a quinoxaline structural unit, such as polyphenylquinoxaline for an electrode material of a proton migration type battery and a composite electrode using such a polymer with an electrically conductive carbon material, which has considerably improved the capacity, however, the improvement is not sufficient to put such a battery to practical use. Moreover, though a composite material of the polymer and an electrically conductive carbon material is preferable for an electrode in terms of the conductivity, studies on the methods did not go into details and the Examples only disclosed methods for mixing a polymer having quinoxaline structural unit and an electrically conductive carbon material. In addition, a problem that the amount of the electrically conductive carbon material required is comparably large, was left unsolved.